1. Field of the Invention
The present invention relates to a power supply circuit suitably used in e.g., a display monitor device.
2. Description of the Related Art
For example, a power supply circuit used in a display monitor device is connected to various kinds of external power supplies from a low voltage of about 100 V to a high voltage of about 220 V. In the power supply circuit, the voltages applied from the connected power supplies are rectified and rectified outputs are applied to a voltage converting circuit (DC--DC converter) so as to obtain a desirable stabilized voltage to be applied to a load.
Use of a step-up chopper circuit for reducing higher harmonic wave current noises and improving power factor is considered in such a power supply circuit. Namely, it is widely known that such a step-up chopper circuit is effective so as to reduce the higher harmonic wave current noises and improve the power factor.
FIG. 1 shows one example of the conventional power supply circuit using a step-up chopper circuit. In FIG. 1, for example, an arbitrary input AC voltage from a low voltage of about 100 V to a high voltage of about 220 V is applied from an external power supply 21 to a rectifying circuit 22. A rectified output of full-wave rectification or half-wave rectification is obtained from both ends of this rectifying circuit 22.
One rectifying output end of the rectifying circuit 22 is connected to a switching element 24 through a coil 23. The other rectifying output end of the rectifying circuit 22 is connected to a common terminal of the switching element 24 through a current waveform detecting circuit 25. Further, a peak value detecting circuit formed of a diode 26 and a capacitor 27 is arranged in parallel with the switching element 24 so that the step-up chopper circuit is formed.
Accordingly, in this power supply circuit, since the above switching element 24 is turned on and off with an arbitrary timing, a voltage boosted in comparison with the rectified output is obtained from the coil 23 instantaneously when the switching element 24 is turned off. A peak value of this output voltage is provided to the capacitor 27. An output DC voltage obtained across both ends of the capacitor 27 is applied to a DC--DC converter 28.
Further, a detecting circuit 30 for an output voltage having a resistor 29 is connected between both ends of the above capacitor 27. The one rectifying output end of the rectifying circuit 22 is connected to a detecting circuit 31 for an effective voltage value. Detecting signals from these detecting circuits 30, 31 and a detecting signal from the above detecting circuit 25 for a current waveform are supplied to a control block 32 for controlling turning-on and turning-off timings of the switching element 24.
For example, loads 33 and 34 are circuits of respective portions of a display monitor device to which a driving voltage from the DC--DC converter 28 is supplied. For example, a standby power supply circuit 35 is used to apply a predetermined standby power to the control block 32 even in a state in which the power supply of the entire device is turned off.
In the above power supply circuit, for example, the turning-on and turning-off timings of the switching element 24 are controlled such that an output DC voltage obtained across both the ends of the capacitor 27 becomes to a predetermined value. Namely, in the above power supply circuit, a boosting voltage ratio is changed by varying an on-off duty of the switching element 24 so that a value of the output DC voltage can be varied.
Therefore, the on-off duty cycle of the switching element 24 is varied by the control block 32 so as to obtain a predetermined output DC voltage with respect to a detecting signal from the above detecting circuit 31 for an effective voltage value. Further, the on-off duty cycle of the switching element 24 is adjusted by the control block 32 by using a detecting signal from the detecting circuit 30 for an output voltage such that this output voltage becomes a desirable value.
Thus, the output DC voltage obtained between both the ends of the capacitor 27 can be adjusted to a predetermined value. In this case, for example, various kinds of applied voltages from a low voltage of about 100 V to a high voltage of about 220 V can be boosted to a fixed output DC voltage of, for example, 360 V. Thus, the monitor device can be stably operated even when any power supply is connected as the external power supply 21.
In accordance with this power supply circuit, the higher harmonic wave current noises are reduced and the power factor is improved by using the step-up chopper circuit so that a preferable power supply circuit can be formed.
However, for example, when the above power supply circuit is mounted to a display monitor device having a maximum consumed power of about 450 W and a power loss is measured, it has been found in this power supply circuit that the power loss is greatly changed in accordance with an input AC voltage as shown in the table of FIG. 2.
Namely, for example, A in FIG. 2 shows a reference value of the power loss when the power supply circuit is formed by double voltage rectification and a capacitor input system without using the step-up chopper circuit. When the input AC voltage is 100 V, the output DC voltage is 262 V. In this case, an output power Pout is 288.2 W when an input power Pin is 318.5 W. Accordingly, a power loss of 30.3 W is caused.
In contrast to this, B and C in FIG. 2 show cases in which the above step-up chopper circuit is used. When the input AC voltage is 100 V (B in FIG. 2), the output DC voltage is 359 V. In this case, the output power Pout is 290.8 W when the input power Pin is 347.5 W. Accordingly, a power loss of 56.7 W is caused. This case shows an increase in loss of 26.4 W in comparison with the case of A in this table.
When the input AC voltage is 230 V (C in FIG. 2), the output DC voltage is 361 V. In this case, the output power Pout is 292.4 W when the input power Pin is 329.5 W. Accordingly, a power loss of 37.1 W is caused. This case shows an increase in loss of 6.8 W in comparison with the case of A in the table.
Thus, in the power supply circuit using the above step-up chopper circuit, a great increase in power loss is caused particularly when the input AC voltage is low. Namely, such a loss is caused by using the step-up chopper circuit when a voltage (solid line) and an electric current (broken line) are formed as shown in, FIG. 3 by turning on and off the switching element 24 and both this voltage and this electric current are not zero.
As mentioned above, for example, when 100 V as an input AC voltage is boosted up to 359 V as an output DC voltage, a boosting voltage ratio is large so that a fall period of the electric current applied to the switching element 24 is made long. Therefore, the power loss caused in this period is increased.
Accordingly, in the case of such an increase in loss, this loss is changed to heat generation in the switching element 24, etc. so that it is necessary to arrange a strong cooling device, etc. for removing this generated heat. When the above output DC voltage is set to be low in accordance with a low input AC voltage, there is a fear that no step-up chopper circuit is operated when a high input AC voltage is supplied.